1. Field of the Invention
The present invention relates to silicon carbide (SiC) densified to a state of closed porosity. The present invention provides methods for forming silicon carbide articles having enhanced fracture toughness without the need for powder beds, gaseous overpressure, sealed crucibles, or other common methods used to limit volatilization.
2. The Prior Art
Prochazka (U.S. Pat. No. 4,004,934) demonstrated that it was possible to sinter SiC without applied pressure by using small additions of boron (B) and carbon (C). His work using cubic SiC was quickly followed by results at The Carborundum Company (U.S. Pat. No. 4,179,299) showing the hexagonal polytypes could also be used as starting materials using the same boron and carbon additives.
Schwetz and Lipp (U.S. Pat. No. 4,230,497) substituted aluminum (Al) for boron, and suggested that aluminum and carbon were superior sintering additives to boron and carbon. U.S. Pat. No. 4,230,497 disclosed a predominantly transgranular fracture mode, which also typically occurs for the Prochazka material.
These silicon carbide materials had their shortcomings, however. All have fracture toughness values of only about 2.5 MPa·m1/2 when measured by the single-edge precracked beam (SEPB) technique, as described in ASTM C 1421-99 (Standard Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature. pp. 641-672 in 1999 Annual Book of Standards, ASTM, Philadelphia, Pa., 1999) incorporated herein by reference.
Suzuki, et al. (U.S. Pat. No. 4,354,991) demonstrated the pressureless sintering of SiC using aluminum oxide (Al2O3) as an additive. Suzuki also had its shortcomings. Due to the reaction between alumina and silicon carbide, which produces SiO, CO, and Al2O gases, powder beds or external powder was needed to generate such gases to retard the decomposition. One attempt to improve on this approach was to lower the temperature of the liquids through the use of a lanthanide aluminate, such as yttria (Y2O3) and alumina as taught by Cutler, et al. (U.S. Pat. No. 4,829,027). These materials have higher toughness (about 4 MPa·m1/2 when measured by the SEPB technique) due to intergranular fracture, which allows for some crack bridging and other toughening mechanisms not operative for SiC which fractures transgranularly. However, they also have inferior corrosion resistance. Additionally, volatility of off gassing species is still an issue at ambient pressure with this and other liquid phase sintering approaches described to date. This is clearly observed with the early work of Omori, et al. (U.S. Pat. Nos. 4,502,983, 4,564,490, and U.S. Pat. No. 4,569,921) where surface segregation of lanthanide compounds was observed, and desired in some instances, when combining solid state sintering additives (Al, B, and C) with lanthanide oxides or their precursors.
Suzuki (U.S. Pat. No. 4,569,922) added AlN in order to form a solid solution with SiC, as well as yttria in order to get an elongated microstructure, which most likely had a fracture toughness above 4 MPa·m1/2 when measured by the SEPB technique. One aspect of this invention is oxygen was desired within the structure. This oxygen, although not specified in the invention, was likely associated with aluminum at grain boundaries and triple points, resulting in intergranular fracture. Elongated grains combined with intergranular fracture gives high fracture toughness. Unfortunately, volatilization was still an issue and gaseous overpressures and/or embedding powders are taught to aid in densification without decomposition.
Ezis (U.S. Pat. No. 5,372,978) showed that equiaxed microstructures could be made with SiC and small additions of AlN. This material, sold under the trade name of SiC—N, by Cercom, Inc. demonstrated good fracture toughness (4.5-5.5 MPa·m1/2 when measured by the SEPB technique) and also has oxygen associated with aluminum at many triple points allowing for its intergranular fracture. However, external pressure was necessary for densification to occur.
Chia, et al. (U.S. Pat. No. 5,298,470) used AlN together with lanthanide oxides and demonstrated high fracture toughness. However, this material, sold under the trade name of SX for a time by The Carborundum Company, also required the use of powder beds to control volatilization. Later, Schwetz et al. (U.S. Pat. No. 6,531,423) used a similar composition but controlled weight loss with a small overpressure (between 2 and 5 atmospheres) prior to reaching closed porosity and then a higher overpressure (95 atmospheres) to aid in densification. Trigg, et al. (U.S. Pat. No. 5,855,841 and U.S. Pat. No. 5,855,842) used a CO overpressure to limit volatilization, which is obviously a less desirable approach due to the toxicity associated with carbon monoxide.
Pujari, et al. (U.S. Pat. No. 6,762,140 and U.S. Pat. No. 6,680,267) combined Y2O3, AlN and/or Al2O3, with boron and carbon to make liquid phase sintered ceramics. Due to their high secondary phase contents, these compositions have volatile species which results in either high weight loss [with the associated problem of deposition of the volatile species elsewhere in the sintering apparatus] or requires weight loss control through powder beds and/or process control.
Silicon carbide is used in a wide range of applications including seals, nozzles, igniters, armor, substrates, semiconductors, mirrors, filters, and impellers. The high erosion, wear, creep, oxidation and chemical resistance of SiC, as well as its electrical properties make it attractive for many products. However, the inability to sinter higher toughness SiC without powder beds and overpressures limits its use. For example, higher toughness SiC would be advantageous for use in armor due to its ability to take multiple hits, as demonstrated by SiC—N. The inability to make this material via a pressureless sintering route makes the material more expensive. Despite the ability to use pressure to densify a diverse range of materials by tailoring their microstructures (see Flinders, et al., “Microstructural Engineering of the Si—C—Al—O—N System,” Ceram. Trans., 178, 63-78 (2005), incorporated herein by reference) pressureless sintering of these same compositions is elusive due to the difficulty in sintering this covalently bonded material.
Flinders et al “High Toughness Silicon Carbide as Armor” J. Am. Ceram. Soc., 88[8], 2217-2226 (2005) (incorporated herein by reference) disclosed the use of Al—Y2O3 as additives in sintering silicon carbide SiC sintered using 0.5-2 wt % Y2O3 and 0.42-1.7 wt. % Al metal were shown to have a high density and a high toughness. However, Flinders' process required hot pressing to accomplish sintering.
The applicants have realized that the problems of weight loss encountered with traditional routes are largely due to the pressure of oxygen. While some oxygen is inevitably present and indeed can contribute to the sintering process, use of Al2O3 or aluminates as the primary source of Al results in too much oxygen being present and hence high weight loss through reaction with the silicon carbide. Provision of Al at least in part in the form of elemental Al enables the use of low quantities of additives, which leads to high SiC contents and consequent high density and low amounts of impurities that might be vulnerable to corrosion. Moving to high silicon carbide contents promotes higher hardness.
It would be an advancement in the art to provide a pressureless sintered silicon carbide-based ceramic with toughness greater than 4 MPa·m1/2, as measured by the SEPB technique, and a density greater than about 3.1 g/cc without using powder beds or overpressures to aid in densification to a state of closed porosity.
It would be a further advancement to provide such a SiC-based ceramic where the amount of additives required in order to maximize the corrosion resistance of these SiC-based ceramics, which fracture predominantly intergranularly, is minimized.
It would be yet another advancement to produce such a toughened silicon carbide-based ceramics that can be manufactured economically for use in applications requiring high wear, erosion, and abrasion resistance. It would be yet another advancement to provide such a SiC-based material that could be tailored for use in multi-hit armor, semiconductor substrates, heat exchangers, microchannel devices, mirrors, wear parts, and other devices presently served by lower toughness silicon carbide. SiC-based material and methods for making SiC-based material that provide some or all of these advancements are disclosed herein.